Juveniles are doubtfully distinguishable from the Common Gallinule of the Americas but juvenile Common Gallinule is said to have a yellower bill and legs and darker smoky-grey face and foreneck. Range reportedly expanded in Northern Europe since the 19th century. Formerly considered conspecific with the Common Gallinule (G. galeata) of North and South America; split into two species by the AOU in 2011. Eurasian Moorhen has its frontal shield narrow and rounded at the top as seen here. Common Gallinule has its frontal shield broader and more squared-off at the top Note also the bright red orbital and extensive yellow on the tip of the lower mandible. Common Gallinule has duller eyes and less yellow on it mandible. There is also a difference in back color. Common Gallinule usually has its back a rich chestnut brown (North American populations), strongest across the upper back where it contrasts with the gray neck and breast. Eurasian Moorhen has a more olive-brown back which blends with the gray and is strongest on the tertials. More reliable than these subtle plumage differences is voice. Common Gallinule has a highly distinctive laughing call. Five subspecies are recognized. This is the widespread nominate race with smaller and darker subspecies confined to subsaharan Africa and various Indian and Pacific Ocean Islands.
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References:
Chesser, R. T., R. C. Banks, F. K. Barker, C. Cicero, J. L. Dunn, A. W. Kratter, I. J. Lovette, P. C. Rasmussen, J. V. Remsen Jr., J. D. Rising, D. F. Stotz and K. Winker. 2011. Fifty-second supplement to the American Ornithologists' Union Check-list of North American Birds. Auk no. 128 (3):600-613.
Taylor, B., Christie, D.A. & Kirwan, G.M. (2018). Common Moorhen (Gallinula chloropus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/53688 on 18 August 2018).